The present invention relates to a load control system for a nuclear power plant of a boiling water reactor (BWR) type.
Heretofore, the BWR nuclear power plant has been operated with a constant output power under a rated load. However, as proportion of power supply by the nuclear power plant becomes increased year after year, there arises a great demand for the operation of the nuclear power plant in a load follow-up control mode, as is in the case of the thermal power plant.
In the hitherto known load control system for the BWR power plant, a recirculated flow is regulated in dependence on a deviation or difference between a set load value and an actual load value. Since the power of the nuclear reactor varies in accordance with regulation of the recirculated flow, the power of the reactor, i.e. the steam pressure supplied to a turbine is correspondingly varied. A governor valve provided at an inlet port of the turbine is controlled in the closing or opening direction in response to increasing or decreasing in the steam pressure with reference to a preset fixed level. As the consequence, the increasing or decreasing in the steam pressure results in the actuation of the governor valve in the closing or opening direction to thereby control correspondingly the output power of an electric generator coupled to the turbine. In other words, the power of the reactor is first controlled, which is then followed by the corresponding control of the turbine system.
In the conventional follow-up control described above, a time lag on the order of several seconds is involved until the output power of the electric generator is varied in response to a change in load. Such time lag may be given in terms of a time constant expression of first order lag which approximates a sum of a time required for the recirculated flow to be varied in accordance with the output signal of the control system and a time required for a change in the thermal output of fuel to be transferred to the coolant within the reactor.
Because the time lag in the load follow-up control described above is never insignificant, resultant loss of generated energy can not be neglected in the load following-up operation to meet a light load demand, for example. Further, in view of conservation of resources which is one of the national problems to be solved, it is desirable that the load following-up operation be carried out without involving any significant time lag.